This invention relates generally to phase change inks and more particularly to additives and the use of additive modified phase change inks in electronic level sensing systems and to the use of additives to encode phase change ink type into the ink composition.
In general, phase change inks are solid at room temperature but exist in the liquid phase at the elevated temperature of an ink jet printer device. At the liquid phase operating temperature droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of a wide variety of printing media, quickly solidify to form a predetermined pattern of solidified ink drops.
Phase change ink is desirable since it remains in a solid phase at room temperature, during shipping, and during long term storage. Also, the problems associated with nozzle clogging due to ink evaporation are essentially eliminated with phase change ink, thereby improving the reliability of ink jet printing. Furthermore, since the ink droplets solidify immediately upon contact with the substrate, migration of the ink along the printing medium is prevented and dot quality is improved.
One of the disadvantages of phase change ink is that it takes time to heat the solid ink to the melting point and then to the higher ink jet operating temperature. U.S. Pat. No. 5,386,224 to Deur et. al. for "Level Sensing Probe System for an Ink Jet Printer" dealt with this problem in an ink level sensing system for an ink jet printer by incorporating the volume of melted ink in the reservoir into an electrical circuit where the volume of melted ink acts as a variable resistor. In this level sensing system high conductance (low resistance) corresponds to a reservoir filled with ink and low conductance (high resistance) corresponds to a reservoir half or less filled with ink. By monitoring the system's conductance, a low ink level can be detected, and the operator alerted, before the reservoir runs dry. If the reservoir does become empty the print head will require re-priming in addition to refilling with an additional loss of operational time and consumption of ink by purging. Difficulties are encountered using this level sensing circuit with many of the embodiments of phase change ink because the class of materials suitable for phase change ink uses natural and synthetic waxes and various resins to obtain the desired transparency, melting point, and microcrystallinity. A characteristic of this class of materials is that they are nonpolar and, as such, do not solvate ions very well, if at all. This microscopic characteristic of the molecules is manifested in the bulk material as low specific conductance, or in other words, they are electrical insulators. The aforementioned level sensing circuit works because of metal ions or amine complexes in the inks.
The use of polymeric colorants in phase change inks present another problem because the trace of specific conductance contributed by impurities is often too small to be interpreted correctly by the electronics. This could result in the operator continuing to be prompted to add ink despite the reservoir already being filled with ink. Eventually the reservoir overflows and the print head and/or the printer is ruined.
These problems are solved by the present invention by providing a means of adjusting upwardly the specific conductance of phase change ink by means of a very small concentration of a specific conductance additive.